In a low-pressure die-casting apparatus, molten metal is forced upwardly through a riser tube having its lower end extending below the level of molten metal in the bath or crucible of a furnace and having its upper end connected to the charging aperture of the die cavity. The molten metal is raised by applying gaseous pressure to the molten metal in the bath; the molten metal rising up the riser tube and into the die cavity, where the metal solidifies. The gaseous pressure is then reduced allowing excess molten metal to fall back down the riser tube to the bath. The casting is then allowed to cool still further after which the die is opened to remove the casting therefrom.
The gaseous pressure used to raise the molten metal from the bath in the furnace into the die cavity, performs two main functions. First, it acts as a pump to transfer the molten metal from the bath up the riser tube and into the die cavity to fill it. The second function is to provide an after-pressure in the molten metal to compact it as it cools in the die cavity. The cooling metal contracts within the die cavity and a certain quantity of metal has to be supplied to the liquid core of the casting to compensate for shrinkage during cooling. As the level of the molten metal in the bath changes due to consumption of the metal or to refilling of the bath, the pressure conditions acting on the molten metal in the bath change.